This invention relates to a method of fabricating a composite metal wire such as an aluminium clad steel wire by extruding a cladding of soft metal around a core of hard metal.
In prior art extrusion processes for fabricating aluminium clad steel wires, a steel core is generally aligned, polished, cleaned or otherwise pretreated and then preheated by conducting electric current thereinto before it is introduced into an extruder. To introduce the core into an extruder and to take it in the form of a composite wire from the extruder, a haul-off unit is located downstream of the extruder to pull the composite wire. Simply pulling the composite wire is not satisfactory. Such a haul-off unit should pull the composite wire at a constant rate. Unless the composite wire is moved at a constant rate, the cladding will vary in thickness so that some products may be rejected. It may be possible to further stretch such clad products using a die. Uniform stretching is difficult and the resulting products will vary in quality. However, this problem has been eliminated by the state-of-the-art haul-off units which can pull a core or composite wire at a constant rate.
It has been found that although a composite wire having a cladding of a uniform thickness is produced, such claddings are liable to peeling or cracking. Furthermore, wires are sometimes broken during extrusion.
The inventors have cooperatively made a research on the above-mentioned problems and have found that although a clad wire is pulled at a constant rate, the tension imparted to a core entering an extruder varies over an unexpectedly wide range and sometimes increases to an extremely high level. Anticipating that this tension variation predominantly causes the above-mentioned shortcomings, the inventors have accomplished this invention.
According to the findings of the inventors, variation of tension to a core will largely affect the adhesion of aluminium or cladding material to the core during extrusion. That is, the adhesion varies as the tension varies. Particularly, an extremely high accidental tension in addition to the normal tension required for pulling will cause breaking of a core.
It will be apparent that such variation of tension to a core occurs during pretreatments including alignment, polishing and cleaning. The core is contacted with an electrode pulley to conduct electric current for preheating, which also causes such variation of tension. If electric current is directly conducted into the core under an increased tension, the core which is thus preheated is stretched to a large extent and tends to be broken. It is also desirable from this point of view that the tension imparted to a core is constant and low. In conventional techniques a core is subject to a tension as high as about 80-90% of the breaking tension of the core. Furthermore, in the case of preheating by the direct electrical conduction, a core tends to oscillate particularly when tension varies along the core. Oscillation will adversely affect the contact of a core with an electrode pulley and sometimes causes spark, damaging the core.
The primary object of this invention is to provide an improved method of fabricating a composite metal wire by introducing a core into an extruder while it is kept under a constant low tension and preheated by conducting electric current thereinto whereby extrusion is carried out in a stable manner and the quality of products is improved.
According to this invention, there is provided a method of fabricating a composite metal wire comprising the steps of subjecting a core of hard metal to pretreatments including alignment, polishing and cleaning and then to preheating, passing the core through an extruder, and thereby extruding a cladding of soft metal around the core, characterized in that the core is introduced into the extruder while the same is kept under a constant low tension of equal to or less than 50%, preferably 5-20% of its breaking tension and preheated by conducting electric current thereinto.
A preferred mode of keeping a core under a constant low tension is to control the tension imparted to the core by locating forcedly driven core feed and haul-off units upstream and downstream of the extruder, respectively, and locating at least one feed-rate correcting dancer roll between these units. In this case, the core feed and haul-off units are synchronously operated at the same rate. The dancer roll serves to compensate for an error in feed rate between the feed and the haul-off units. This arrangement is very advantageous in that the tension imparted to a core may be reduced to a value of not more than 50% of the breaking tension of the core, although the conventional techniques require to apply a tension of about 80-90% of the breaking tension.
The use of double capstans as the core feed unit is very advantageous in practice because a core is less contaminated at its surface when compared with the use of a pair of endless belts which clamp a core therebetween and carry it forward with the aid of friction. The double capstan system is preferable to a single capstan system because the latter system requires to wind a core around the capstan barrel several times. Such winding is unnecessary and a core is less damaged or contaminated in the former system. Contamination of a core at this stage not only renders the preceding cleaning step vain, but also adversely affects the adhesion of the core to a cladding metal.
Preheating of a core is achieved by conducting electric current thereinto according to a preferred aspect of this invention. Since the use of a number of pulleys for current conduction as such causes a core to oscillate and tension to vary, it is recommended to use a minimum number of pulleys for current conduction. It is therefore preferable to use a die box of the extruder as one of electrodes for conducting current into a core. It is also preferable to use a fixed or idler roll of the feed-rate correcting dancer roll assembly as another electrode for electrical conduction. The afore-mentioned concept of using the die box of the extruder as an electrode for electrical conduction is very convenient since the core is effectively and economically heated and electrical conduction is stable so that spark generation is substantially eliminated. This concept is also desirable from a point of view of preventing substantial oxidation due to heating.
The hard and soft metals which can be used herein are selected from the group consisting of steel, copper, aluminium, zinc, magnesium, lead, tin, cadmium and alloys thereof. It will be easy for one skilled in the art to select two materials among them and determine which one should be used as the hard or the soft metal by comparing the workability of the two. Among products fabricated by the present method most preferred is an aluminium clad steel wire.